Commercially available hardcoat coatings possessing physical and chemical properties that are useful in a wide variety of applications are well known in the art. For purposes of the present specification, a hardcoat (a.k.a., hard coating) will refer to a coating that exhibits good mechanical properties, such as, flexibility, scratch-resistance and abrasion-resistance.
Conventional thermal cure silicone hardcoats are often too stiff and/or brittle to be flexed or bent beyond a 1.8 percent “strain” with out fracturing, i.e., cracking or delaminating from a substrate. This limitation has restricted the use of silicone hardcoat materials to applications were the substrate is rigid enough to keep deformation, at the surface, to less than 1.8 percent strain. As a result of this constraint, the use of thermal cure silicone hardcoats is precluded from flexible plastic films and polyethylene terephthalate (PET) films in particular, due to the tendency of the coating to fracture when the plastic film is flexed during handling.
Typically, harder more highly crosslinked hardcoats exhibit improved scratch resistance, however, they are less flexible and much more susceptible to chipping or thermomechanical cracking due to embrittlement of the film resulting from a high crosslink density. Conversely, softer, less crosslinked hardcoat, while not as prone to chipping or thermomechanical cracking, is susceptible to scratching, waterspotting, and acid etch due to a low crosslink density of the cured film.
Additionally, thermally cured silicone hardcoats are treated with other materials, e.g., UV absorbing materials to prevent degradation of the underlying polymer substrate from exposure to ultraviolet (UV) light, which effects the adhesion of the coating to the substrate and the mechanical and optical properties of the substrate.
It is therefore an object of the present invention to provide a thermally cured silicone hardcoat composition having a high level of flexibility, abrasion resistance and, in general, improved resistance to cracking under exposure to thermal and/or mechanical stresses.